Because hydrofluorocarbons (HFCs) do not deplete the ozone layer, they are becoming popular substitutes for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) for use as heat transfer agents, blowing agents, and propellants. HFCs are typically prepared by fluorinating a chlorinated organic compound with a fluorination agent such as hydrogen fluoride in the presence of a fluorination catalyst. This reaction may be conducted in either the liquid or gas phase. Generally, the liquid phase fluorination is preferred because the reaction is controlled at relatively lower temperatures which results in less by-product formation due to decomposition.
Liquid phase fluorination, however, uses and generates corrosive compounds, such as, for example, hydrogen fluoride, hydrogen chloride, and catalysts, which form superacids. These superacids tend to corrode the reactor in which the reaction is conducted, even reactors comprised of corrosion-resistant materials such as Inconel 600, NAR25-50MII, Hastelloy C, Hastelloy G-30, duplex stainless steel, and Hastelloy C-22. Corrosion of the reactor compromises the structural integrity of the reactor and reduces its useful life. Therefore, a need to minimize reactor corrosion exists.
One method of reducing such corrosion is taught in Japanese Kokai Patent Application Publication No. 233102(1995). In this publication, a method is disclosed for the liquid phase fluorination of a chlorinated organic compound in a reactor made or lined with a fluorine resin. The method involves gaseous feeds of hydrogen fluoride and chlorinated organic compound. Because the process is restricted to gaseous feed streams, it is limited in the type of HFCs it can produce. Chlorinated organic compounds having two or more carbon atoms tend to decompose before reaching their gaseous state. For example, pentachloropropane tends to decompose significantly at temperature greater than 90.degree. C. while its boiling point is about 190.degree. C. Thus, as a practical matter, the process disclosed in this publication can only be used to produce fluorinated methanes.
The aforementioned Japanese publication also states that when heat transfer through the reactor is necessary, which is usually the case in liquid phase fluorination, the fluorine resin liner should be applied using a molding method. The only molding method identified therein is rotary-baked molding.
Generally, reactors having a molded liner, such as a rotary-baked or sprayed-on liner, are not suitable for large-scale commercial production. Reactors having such liners must be baked in large kilns or ovens, which are expensive and frequently unavailable. Indeed, fitting a large reactor, for example, greater than about a 1,000 gallons, with a baked liner is impractical.
A molded liner not only imposes practical limitations on the reactor, but also introduces structural limitations. It has been found that molded liners tend to be permeable and, under high pressures and over time, reactants tend to penetrate the liner and develop pressure between the liner and the reactor wall. This pressure causes the liner to blister, and eventually the liner comes loose. The problem of liner penetration is exacerbated by the absence of weep holes in a molded-liner reactor. Ordinarily, weep holes allow reactants that penetrate the liner to escape from the reactor. A molded liner, however, generally cannot be used in a reactor with weep holes. When applying a molded liner, a fluid fluoropolymer is applied to the reactor wall, and, thus, holes in the reactor wall will result in holes in the molded liner. Holes in the liner obviously compromise the reactor's ability to be pressurized. Therefore, while a rotary-baked, fluorine-resin liner may minimize reactor corrosion, its structural limitations nevertheless limit the reactor's useful life.
Therefore, a need exists for a commercially viable method of producing a wide range of HFCs while minimizing reactor corrosion. The present invention fulfills this need among others.